Optical recording and reproducing apparatus for tracking wobbling guide grooves

ABSTRACT

An optical recording and reproducing apparatus uses a laser to form one or more spots on an information recording medium preformatted with wobbling guide grooves, and detects tracking error. If the differential push-pull method of detecting tracking error is used, a center spot is flanked by satellite spots distant by an odd multiple of half the repeating period of the wobble. If the three-beam method is used, the distance is an odd multiple of one-fourth the repeating period of the wobble. If the push-pull method is used, a split photodetector generates a pair of electrical signals. The sum and difference of these signals are filtered, then synchronously detected, and the result is combined with the difference signal to generate a corrected tracking error signal.

BACKGROUND OF THE INVENTION

This invention relates to optical recording and reproducing apparatusemploying media preformatted with wobbling guide grooves, moreparticularly to the detection of tracking error in such apparatus.

Optical reproducing apparatus is already widely manufactured and sold inthe form of compact disc players, and apparatuses are now becomingavailable that can both reproduce and record information, usingwrite-once-read-many media or overwritable media. In both types ofapparatuses information is read from (or written on) spiral orconcentric tracks by means of a laser beam. The beam is kept on-track bya system that detects tracking error, and a servo system that correctsthe detected tracking error.

A variety of methods are used to detect tracking error. In thewell-known push-pull method, the laser beam is reflected from the mediumto a split photodetector. Signals from the two halves of the detectorare added to obtain the reproduced information signal, and mutuallysubtracted to obtain a tracking error signal.

In another method known as the three-beam, triple-beam, tri-beam, ortwin-spot method, three laser beams are focused onto the medium,creating three spots that are aligned at an angle to the tracks. Thecenter spot is used for reading or writing information. The other twospots, referred to as satellite spots, are reflected to two separatephotodetectors, and the difference between the photodetector outputsignals is used as the tracking error signal.

Still another method, known as the differential push-pull method, alsoemploys three spots, but reflects each spot to a split photodetectorthat generates a difference signal. The tracking error signal isobtained by adding the two difference signals from the satellite spotsand subtracting the difference signal from the center spot.

These three methods were originally developed for use with media havingregular circular or spiral tracks, but there has been a recent movementtoward media on which the tracks are formed in wobbling guide grooves.For example, standards for compact disc (CD) media have been proposed inwhich absolute address information and information for constant linearvelocity control are encoded in the wobble.

A general problem arising with such media is that if the tracking errordetection system detects the wobble, the tracking servo may attempt totrack the wobble, thereby defeating the purpose of the wobble. In thepush-pull method, for example, if the servo system tracks the wobbleaccurately, the information encoded in the wobble is lost. A similarproblem occurs in the differential push-pull method when the spacingbetween the three spots is one-fourth the repeating period of thewobble.

A somewhat different problem can occur in the three-beam method. If thetwo satellite spots are separated by a distance equal to the repeatingperiod of the wobble, the servo system tracks the wobble at thelocations of the satellite spots, thereby doubling the amplitude of thewobble at the central read/write spot. In recording, this leads topoorly formed pits; in reproducing, it degrades the quality of thereproduced signal.

The wobble-tracking problem is related to the problem of trackingoffset, which can be caused by tilting of the rotating medium ortrack-following movement of the objective lens. This problem occursparticularly when the push-pull method is used. Previous attempts tocorrect offset by means of sensors that detect media tilt andobjective-lens movement have been unsatisfactory, in part because theyincrease the complexity and cost of the apparatus. Attempts to avoid thesecond type of offset by moving the optical system as a whole instead ofjust the objective lens also increase the cost of the apparatus, becausea more powerful actuator is needed.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to generate atracking error signal with reduced wobble and offset components.

Another object of the invention is to reduce the cost of opticalrecording and reproducing apparatus.

According to one aspect of the invention, an optical recording andreproducing apparatus record or reproduce information on or frominformation recording media having guide grooves that wobble with acertain repeating period. The apparatus comprises a laser for emittingone or more laser beams and an optical system for focusing the beam orbeams to form a center spot and two satellite spots on theinformation-recording medium. The center spot and satellite spots arealigned in a straight line at an angle to the guide grooves. The centerspot is disposed midway between the satellite spots and is distant fromeach satellite spot by an odd multiple of substantially one-half therepeating period of the wobble. Tracking error is detected by thedifferential push-pull method.

According to another aspect of the invention, the center spot is distantfrom each satellite spot by an odd multiple of substantially one-fourththe repeating period of the wobble and tracking error is detected by thethree-beam method.

According to still another aspect of the invention, the optical systemforms a single spot on the information-recording medium, and adifference signal is generated by the push-pull method. A bandpassfilter extracts the wobble component of this difference signal. Thewobble component and the reproduced information signal are input to asynchronous detector that produces a correction signal. A correctioncircuit combines the correction signal with the difference signal toproduce a tracking error signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the optical system and related parts of anovel optical recording and reproducing apparatus employing thedifferential push-pull method.

FIG. 2 is a plan view of the information recording medium in FIG. 1,showing the positions of the spots.

FIG. 3 is a schematic diagram of the tracking error detection circuit ofthe apparatus in FIG. 1.

FIG. 4 is a simplified sectional view of the optical system,illustrating offset caused by tilt of the information recording medium.

FIG. 5 is another simplified sectional view of the optical system,illustrating offset caused by movement of the objective lens.

FIG. 6 is another plan view of the information recording medium.

FIG. 7 is a sectional view of the optical system and related parts of anovel optical recording and reproducing apparatus employing thethree-beam method.

FIG. 8 is a plan view of the information recording medium in FIG. 7,showing the positions of the spots.

FIG. 9 is a schematic diagram of the tracking error detection circuit ofthe apparatus in FIG. 7.

FIG. 10 is a sectional view of the optical system and related parts ofanother novel optical recording and reproducing apparatus employing thethree-beam method.

FIG. 11 is a sectional view of the optical system and a schematicdiagram of the tracking error detection circuit of a novel opticalrecording and reproducing apparatus employing the push-pull method.

FIG. 12 is a plan view of the information recording medium in FIG. 11.

FIG. 13 is a schematic diagram illustrating a variation of thesynchronous detector in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Novel optical recording and reproducing apparatus employing thedifferential push-pull method, the three-beam method, and the push-pullmethod of tracking error detection will be described with reference tothe attached drawings. The drawings are illustrative and do not restrictthe scope of the invention, which should be determined solely from theappended claims.

Referring to FIG. 1, a first novel optical recording and reproducingapparatus comprises a disc-shaped information recording medium 2 thatrotates in a plane at right angles to the page, and a semiconductorlaser 4 that emits a laser beam 5. An optical system 6 diffracts andfocuses the laser beam 5 to form a center spot 8 and two satellite spots10 and 12 on the information recording medium 2. Three splitphotodetectors 14, 16, and 18 receive light reflected from theinformation recording medium 2 and convert the light to electricalsignals that are input to a tracking error detection circuit to be shownlater.

The optical system 6 comprises a collimator lens 20, a diffractiongrating 22, a polarizing beam splitter 24, a quarter-wave plate 26, anobjective lens 28, and a convergent lens 30. The collimator lens 20converts the divergent laser beam 5 emitted by the laser 4 to a parallelbeam. The diffraction grating 22 separates this parallel beam into threebeams: a zeroth-order diffracted beam 32, and two first-order diffractedbeams 34 and 36. These three beams pass through the polarizing beamsplitter 24 and the quarter-wave plate 26 and are focused by theobjective lens 28 onto the information recording medium 2, thezeroth-order diffracted beam 32 forming the center spot 8 and thefirst-order diffracted beams 34 and 36 forming the two satellite spots10 and 12.

After reflection from the information recording medium 2, the threebeams 32, 34, and 36 re-enter the optical system 6, pass through theobjective lens 28 and the quarter-wave plate 26, are reflected in thepolarizing beam splitter 24, and are directed by the convergent lens 30onto the three split photodetectors 14, 16, and 18. The zeroth-orderdiffracted beam 32 is received by the first split photodetector 14,while the two first-order diffracted beams 34 and 36 are received by thesecond and third split photodetectors 16 and 18.

FIG. 2 is a plan view showing part of the information recording medium2. When the information recording medium 2 is manufactured, it ispreformatted with guide grooves 38 that wobble with a substantiallyconstant repeating period W around center lines 40. The wobbling issubstantially in phase between adjacent grooves. The center lines 40 aredrawn as double lines representing where the edges of the guide grooves38 would be if the guide grooves did not wobble. The wobble is greatlyexaggerated for clarity in the drawings; the actual amplitude of thewobble is nominally one twenty-fifth of the spacing P of the guidegrooves 38. This spacing P will be referred to below as the track pitch.

Information is recorded in the guide grooves 38 in the form of pits 41.The areas between the guide grooves 38 are referred to as land areas 42.As shown in the diagram, the spatial frequency of the pits 41 is muchhigher than the spatial frequency of the wobble. The pits 41 need notfollow the wobble of the guide grooves 38; normally they follow thecenter lines 40 as shown in the drawing. Due to the exaggerated wobblein the drawings, some pits are shown as departing from the guidegrooves, but in practice, such departure would be either nonexistent orextremely slight.

The center spot 8 and the two satellite spots 10 and 12 are aligned in astraight line at an angle to the guide grooves 38 (more precisely, at anangle to their center lines 40). The center spot 8 is disposed midwaybetween the two satellite spots 10 and 12, at a distance L from eachsatellite spot in the horizontal direction in the drawing, i.e., thelongitudinal direction of the guide grooves. L is equal to an oddmultiple of half the repeating period W. In the drawing L is equal toone-half W, but the invention can be practiced with other odd multiplessuch as three-halves W, five-halves W, and so on.

L is the distance between the spots in the direction of the guidegrooves 38 (the horizontal direction in the drawing). The two satellitespots 10 and 12 are also displaced from the center spot 8 in thevertical direction in the drawing, i.e., in the lateral direction of theguide grooves, by a distance Q equal to one-half the track pitch P. Q ismuch smaller than L, so the true distance of the center spot 8 from thesatellite spots 10 and 12 is substantially equal to L.

FIG. 3 is a schematic diagram illustrating the three photodetectors 14,16, and 18 and the tracking error detection circuit 43 to which they areconnected. As shown, each split photodetector comprises two separatelight-receiving surfaces that generate two separate electrical signals.

The tracking error detection circuit 43 comprises four differentialamplifiers 44, 46, 48, and 50, a summing amplifier 52, and twovariable-gain amplifiers 54 and 56. The differential amplifier 44receives the two electrical signals D₁ and D₂ output by the first splitphotodetector 14 and generates a first difference signal E₀. Thedifferential amplifier 46 receives the two electrical signals D₃ and D₄output by the second split photodetector 16 and generates a seconddifference signal E₁. The differential amplifier 48 receives the twoelectrical signals D₅ and D₆ output by the third split photodetector 18and generates a third difference signal E₂, which is amplified by thevariable-gain amplifier 54. The summing amplifier 52 adds the amplifiedthird difference signal E₂ to the second difference signal E₁. Thevariable-gain amplifier 56 amplifies the resulting sum E₃. Thedifferential amplifier 50 subtracts the amplified sum E₃ from the firstdifference signal E₀ to generate a tracking error signal TE.

The tracking error signal TE is supplied to a tracking servo system, notshown in the drawings, that positions the three spots 8, 10, and 12 bymoving the objective lens 28 in FIG. 1, or by moving the entire opticalsystem 6 together with the laser 4 and the split photodetectors 14, 16,and 18. The tracking servo system is adapted to adjust the positions ofthe spots 8, 10, and 12 so as to reduce the value of the tracking errorsignal TE to zero.

The tracking error detection circuit 43 also supplies a constant linearvelocity control signal CLV to a spindle servo system, not shown in thedrawings, that controls the rotation of the information recording medium2. The first difference signal E₀ can be used as this constant linearvelocity control signal CLV, as shown in the drawing.

Another summing amplifier 57 receives the two electrical signals D₁ andD₂ output from the first split photodetector 32 and generates their sumas a reproduced information signal RF. This signal RF is supplied tosignal-processing circuits, not shown in the drawings, that reproducethe information encoded in the pits 41.

Next the difference signals E₀, E₁, and E₂ will be described in moredetail.

The three difference signals E₀, E₁, and E₂ have four components: ahigh-frequency information component due to the pits 41; alower-frequency wobble component due to the wobble of guide grooves 38;a still lower-frequency tracking error component due to displacement ofthe center spot 8 from the center line 40 of the guide groove 38 beingfollowed; and an offset component generated by such factors as tilt ofthe information recording medium 2 and movement of the objective lens28.

FIG. 4 illustrates how tilt of the information recording medium 2 cangenerate offset. For clarity, the diffraction grating 22 and thequarter-wave plate 26 are omitted and only the zeroth-order diffractedbeam 32 is shown. The center spot 8 is exactly centered in a guidegroove 38. If the information recording medium 2 were not tilted,reflected light from the center spot 8 would follow the dashed arrowsand illuminate the two halves of the photodetector 14 equally. Becauseof the tilt, however, the reflected light follows the solid lines andilluminates the two halves of the photodetector 14 unequally, causing adc offset in the difference signal.

FIG. 5 illustrates how displacement of the objective lens 28 cangenerate offset. This problem occurs when track following is performedby moving the objective lens 28 without moving the photodetectors. Ifthe objective lens 28 is centered over the guide groove 38 beingtracked, as indicated by dashed lines, reflected light follows thedashed arrows and illuminates the two halves of the photodetector 14equally. If the objective lens 28 has been moved downward in thedrawing, however, as indicated by solid lines, reflected lightilluminates the two halves of the photodetector 14 unequally. Theresulting offset in the difference signal changes with movement of theobjective lens 28, but the change becomes appreciable only after theobjective lens 28 has moved much farther than the track pitch P. Overreasonable periods of time, such as during the following of a singletrack in FIG. 2, this offset can also be treated as a dc component ofthe difference signal.

Next the use of the constant linear velocity control signal CLV will bedescribed.

The spindle servo system has a bandpass filter that rejects the dcoffset component, the low-frequency tracking error component, and thehigh-frequency information component of the constant linear velocitycontrol signal CLV. The spindle servo thus responds only to the wobblecomponent. The spindle servo is adapted to control the rotation of theinformation recording medium 2 so that the wobble component has aconstant center frequency of, for example, 22.05 kilohertz as in theproposed compact-disc standard.

Although the repeating period W of the wobble is substantially constant,it is not exactly constant; it is modulated slightly to encode absoluteaddress information. This modulation is detected as a frequencymodulation of the constant linear velocity control signal CLV with amaximum deviation of ±1 kilohertz. Thus during normal operation thewobble has a frequency of 22.05±1 kilohertz.

Next the tracking error signal TE will be described. The tracking servosystem has a low-pass filter that rejects the high-frequency informationcomponent of the tracking error signal TE, so the high-frequencyinformation components of the difference signals E₀, E₁, and E₂ will beignored in this description.

Taken together, the tracking error and wobble components can beapproximated by a sine function of the deviation of the spot from theguide groove. Referring to FIG. 6 (in which the pits 41 are omitted forclarity), let U be the deviation of the guide grooves 38 from the centerline 40 at the location of the center spot 8. Since the satellite spots10 and 12 are separated from the center spot 8 by a distance L equal tohalf the repeating period W of the wobble, at the satellite spots 10 and12 the guide grooves 38 deviate by an equal amount -U in the oppositedirection. Let X be the tracking error, i.e. the deviation of the centerspot 8 from the center line 40 (in the drawings X is zero). Then thethree difference signals can be represented as follows:

    E.sub.0 =A.sub.0 sin [2π(X-U)/P]+B.sub.0

    E.sub.1 =A.sub.1 sin [2π(X+Q+U)/P]+B.sub.1

    E.sub.2 =A.sub.2 sin [2π(X+Q-U)/P]+B.sub.2

The constants A₀, A₁, and A₂ are the amplitudes of the tracking errorplus wobble components of the three difference signals and B₀, B₁, andB₂ are their offset components. Since Q=P/2 and hence 2π Q/P=π, theformulas for E₁ and E₂ can be simplified as follows.

    E.sub.1 =-A.sub.1 sin [2π(X+U)/P]+B.sub.1

    E.sub.2 =-A.sub.2 sin [2π(X+U)/P]+B.sub.2

From FIGS. 4 and 5 it can be understood that offset affects all threedifference signals in the same way; that is, the amplitudes and offsetsare related as follows:

    A.sub.0 /B.sub.0 =A.sub.1 /B.sub.1 =A.sub.2 /B.sub.2

The gain G₁ of the variable-gain amplifier 54 is set to A₁ /A₂. Thesignal E₃ can therefore be expressed as follows:

    E.sub.3 =E.sub.1 +G.sub.1 E.sub.2 =-2A.sub.1 sin [2π(X+U)/P]+2B.sub.1

The gain G₂ of the variable-gain amplifier 56 is set to A₀ /(2A₁). Thetracking error signal TE is therefore: ##EQU1## Since the wobble U ismuch smaller than the track pitch P, cos(2π U/P) is substantially equalto unity. To a close approximation, TE is therefore expressible asfollows:

    TE=2A.sub.0 sin (2πX/P)

As a sine function of X, this tracking error signal TE accuratelyrepresents the tracking error, without offset or wobble. The trackingservo therefore tracks the center line 40 accurately, resulting in ahigh-quality constant linear velocity control signal CLV, a high-qualityreproduced information signal RF during reproduction, and well-formedpits during recording.

In the case of a compact disc with a constant linear velocity between1.2 and 1.4 meters per second in which the wobble frequency is 22.05±1kilohertz, to be equal to half the repeating period of the wobble, theseparation L between the center spot 8 and the satellite spots 10 and 12should be 24 to 35 microns. The invention is not restricted to thisparticular range of values however, odd multiples of half the repeatingperiod of the wobble also being usable.

FIG. 7 shows a second novel optical recording and reproducing apparatusthat uses the three-beam method. The optical system is the same asbefore except that the collimator lens 20 is disposed below thepolarizing beam splitter 24, a divergent lens 58 is used instead of theconvergent lens 30 in FIG. 1, and an additional cylindrical lens 60 isprovided. The zeroth-order diffracted beam 32 reflected from the centerspot 8 is received by a quadrant photodetector 62, which providessignals for information reproduction and focus control. The twofirst-order diffracted beams 34 and 36 reflected from the satellitespots 10 and 12 are received by a first photodetector 64 and a secondphotodetector 66.

FIG. 8 is a plan view of part of the information recording medium 2,showing the three spots in two positions: one (in the upper part of thedrawing) in which the center spot 8 is disposed at a point of minimumwobble, and another (in the lower part of the drawing) in which thecenter spot 8 is disposed at a point of maximum wobble. Of course thethree spots 8, 10, and 12 are in only one position at a time. The lowerposition results from the upper after a rotation of the informationrecording medium 2 in the direction of the arrow X.

The distance of the two satellite spots 10 and 12 from the center spot 8is exactly half what it was in FIG. 2. The displacement Q' in thevertical direction is one-fourth the track pitch P. The separation L' inthe horizontal direction in the drawing is one-fourth the repeatingperiod W of the wobble. In the case of media rotating with a constantlinear velocity of 1.2 to 1.4 meters per second and a wobble frequencyof 22.05±1 kilohertz, the separation L' should be 11 to 18 microns.

FIG. 9 shows the tracking error detection circuit 67 to which the firstand second photodetectors 64 and 66 are connected. The firstphotodetector 64 produces a first electrical signal E₄ that is fed tothe non-inverting input of a differential amplifier 68. The secondphotodetector 66 produces a second electrical signal E₄ that isamplified by a variable-gain amplifier 70 and fed to the inverting inputof the differential amplifier 68. The output of the differentialamplifier 68 is a tracking error signal TE'.

To avoid obscuring the invention with unnecessary detail, theelectronics for processing signals from the quadrant photodetector 62and generating the reproduced information signal and constant linearvelocity control signal are omitted.

The first and second electrical signals E₄ and E₅ can be approximated bycosine functions, with amplitudes A₄ and A₅, of the displacement of thespot from the guide groove, plus offset components B₄ and B₅. Since thesatellite spots 10 and 12 are mutually separated by half the repeatingperiod of the wobble, the wobble U' at the satellite spot 12 is equaland opposite to the wobble -U' at the satellite spot 10.

    E.sub.4 =A.sub.4 cos [2π(X-Q'+U')/P]+B.sub.4

    E.sub.5 =A.sub.5 cos [2π(X+Q'-U')/P]+B.sub.5

The gain G₃ of the variable-gain amplifier 70 is set to the amplituderatio A₄ /A₅, which equals the offset ratio B₄ /B₅. The tracking errorsignal TE' is accordingly: ##EQU2## Q' is P/4, and U' is much smallerthan P, so 2π (Q'-U')/P is close to π/2 and sin [2π(Q'-U')/P] issubstantially equal to unity. Therefore, to a close approximation:

    TE'=2A.sub.4 sin (2π X/P)

This tracking error signal TE' is free of offset and substantially freeof wobble, resulting in the same benefits as in the first novelapparatus: accurate tracking of the center line 40, a high-qualityconstant linear velocity control signal, a high-quality reproducedinformation signal during reproduction, and well-formed pits duringrecording.

It is not necessary for the spacing between the spots to be one-fourththe repeating period W of the wobble as in FIG. 8. A spacing ofthree-fourths W (33 to 54 microns for media rotating with a constantlinear velocity of 1.2 to 1.4 meters per second and a wobble frequencyof 22.05±1 kilohertz) gives the same effect, as do other odd multiplesof one-fourth W.

The apparatus in FIGS. 1 and 7 uses a diffraction grating to producethree laser beams, but the invention is not restricted to this means ofbeam production. Instead of a diffraction grating, it is possible toemploy a monolithic semiconductor laser array that emits three laserbeams from a single lasting region, or a hybrid semiconductor laserarray having three lasing regions, each of which emits a single laserbeam.

FIG. 10 shows an apparatus that employs a laser array 71 of this type toemit three laser beams 32, 34, and 36. Other components are the same asin FIG. 7, so a description will be omitted. If the beams are disposedso that the interval between the three spots 8, 10, and 12 is an oddmultiple of one-fourth the repeating period of the wobble, the sameeffect is obtained as in FIG. 7. In addition, the cost of the apparatusis reduced because no diffraction grating is needed in the opticalsystem.

A similar type of laser array 71 can be used when the differentialpush-pull method is employed. In this case the beams should of course bedisposed so that the intervals between the spots are an odd multiple ofone-half the repeating period of the wobble.

FIG. 11 shows a novel optical recording and reproducing apparatus thatuses the push-pull method. As before, the information recording medium 2has a wobbling guide grooves 38 and is controlled to rotate with aconstant linear velocity at which the wobble frequency is, for example,22.05±1 kilohertz.

This method employs only a single laser beam 5 and spot 72, so theoptical system requires no diffraction grating or other means ofproducing multiple beams and spots. FIG. 11 illustrates a simplifiedrepresentation of the optical system, showing only the collimator lens20, the polarizing beam splitter 24, and the objective lens 28.

The push-pull method employs a single split photodetector 74 thatreceives reflected light from the spot 72 and generates a pair ofelectrical signals D₇ and D₈. These electrical signals D₇ and D₈ are fedto a differential amplifier 76 and a summing amplifier 78. The summingamplifier 78 generates a sum signal that can be used as the reproducedinformation signal RF. The differential amplifier 76 generates adifference signal E₆ that can be used as the constant linear velocitycontrol signal CLV.

The difference signal E₆ is fed to a bandpass filter 80 with cutofffrequencies of, for example, 20 and 30 kilohertz that extracts thewobble component D of the difference signal E₆. This wobble component Dand the sum signal from the summing amplifier 78 are fed to asynchronous detector 81 comprising a low-pass filter 82, an amplifier84, a multiplier 86, and another low-pass filter 88.

The low-pass filter 82 receives the sum signal RF and rejects itshigh-frequency information component. The cutoff frequency of thelow-pass filter 82 is, for example, 30 kilohertz. The amplifier 84amplifies the output of the low-pass filter 82 and sends the filtered,amplified sum signal S to the multiplier 86. The multiplier 86 alsoreceives the wobble component D of the difference signal E₆ from thebandpass filter 80. The multiplier 86 multiplies D by S and sends theirproduct M₀ to the low-pass filter 88.

The low-pass filter 88 has a cutoff frequency less than the wobblefrequency. If the wobble frequency is 22.05±1 kilohertz, the cutofffrequency of the low-pass filter 88 is, for example, less than 20kilohertz. The output from the low-pass filter 88 is a correction signalM₁ that is combined with the difference signal E₆ in a correctioncircuit 89 to generate a corrected tracking error signal CTE.

The correction circuit 89 comprises an amplifier 90 that amplifies thecorrection signal M₁, and a differential amplifier 92 that subtracts theamplified correction signal received from the amplifier 76 from thedifference signal E₆ to generate the corrected tracking error signalCTE.

FIG. 12 is a plan view of part of the information recording medium 2,showing the spot 72 disposed on the center line 40 of a guide groove 38so that the tracking error X is zero. As before P is the track pitch.The letter V represents the maximum amplitude of the wobble, which isexaggerated in the drawing but is substantially one twenty-fifth of thetrack pitch P.

Tracking error detection by this optical recording and reproducingapparatus can be described mathematically as follows.

The difference signal E₆ output from the differential amplifier 76 hasthe same form as the signal E₀ discussed earlier:

    E.sub.6 =A sin [2π(X-U)/P]+B

where A is the amplitude of the tracking error component, B is an offsetcomponent, U is a wobble, and X is the tracking error. The wobble U canbe expressed as follows:

    U=V sin (2π ft)

where f is the frequency of the wobble (22.05±1 kilohertz, for example)and t represents time.

The bandpass filter 80 rejects frequency components other than thewobble-frequency component. Furthermore, U is much smaller than P, sosin(2π U/P) is substantially equal to U/P. The signal D output by thebandpass filter 80 therefore has the form: ##EQU3##

The low-pass filter 82 eliminates the high-frequency informationcomponent of the reproduced information signal RF, so the signal Soutput by the amplifier 84 has the form:

    S=A' cos [2π(X-U)/P]+B'

where A' is the amplitude of the tracking error component and B' is anoffset component. The product M₀ of D and S can be calculated asfollows: ##EQU4## since the cutoff frequency of the low-pass filter 22is less than the wobble frequency f, low-pass filtering of M₀ eliminatesDB' and DA' cos(2πX/P) and produces the following correction signal M₁ :

    M.sub.1 =K sin (2π X/P)

where

    K=-(2π V/P)(2π V/P)AA'/2

K is a constant, so the correction signal M₁ is expressed as a sinefunction of the tracking error X. The correction signal M₁ itself couldtherefore be used as a tracking error signal, but it would not be veryaccurate because of the small value of the constant K.

However, when E₆ is equal to zero, the correction signal M₁ gives anapproximate indication of the size of the offset component B. If thegain G₄ of the amplifier 76 is adjusted so that G₄ M₁ =B in this case,the unwanted offset component can be removed from the corrected trackingerror signal CTE generated by the differential amplifier 78 when E₆ iszero, thereby causing the tracking servo to move the spot 72 closer tothe center line 40. The correction signal M₁ is further morewobble-free, so combining M₁ and E₆ in this way reduces the relativemagnitude of the unwanted wobble component in CTE. Use of the correctedtracking error signal CTE thus improves both the accuracy andreliability of track following operations.

While the corrected tracking error signal CTE in FIG. 11 is notnecessarily as accurate as the tracking error signals TE and TE'generated by the novel apparatus in FIGS. 1 to 10, the apparatus in FIG.11 has the advantage of lower cost due to a simpler optical system.Moreover, all of the apparatus shown in FIGS. 1 to 12 has the advantagethat by eliminating or reducing the offset component of the trackingerror signal, it eliminates or reduces the need for sensors to detecttilt of the information recording medium 2 or movement of the objectivelens 28, and increases the range over which track following can becarried out by moving the objective lens 28 without moving the entireoptical system.

The synchronous detector 81 and correction circuit 89 are not restrictedto the configurations shown in FIG. 11. For example, the low-pass filter82 in FIG. 11 can be replaced by a bandpass filter 94 identical to thebandpass filter 80, as shown in FIG. 13. The preceding analysis of thesignal S is unchanged by this replacement.

Other modifications that will be obvious to those skilled in the art canbe made in the apparatus shown in FIGS. 1, 3, 7, 9, 10, and 11 withoutdeparting from the spirit and scope of the present invention as setforth in the following claims.

What is claimed is:
 1. An optical apparatus for recording or reproducinginformation on or from an information recording medium having groovesthat are disposed at a certain pitch and that wobble with a certainrepeating period, said apparatus comprising:a laser for emitting atleast one laser beam; an optical system for receiving said laser beamand forming a center spot and two satellite spots on said informationrecording medium, said center spot and said two satellite spots beingaligned in a straight line at an angle to said grooves with said centerspot midway between said two satellite spots, said center spot beingdistant from each of said two satellite spots by an odd multiple ofsubstantially one-fourth said repeating period in a longitudinaldirection of said grooves; a first photodetector for receiving reflectedlight from one of said two satellite spots and generating a firstelectrical signal; a second photodetector for receiving reflected lightfrom another of said two satellite spots and generating a secondelectrical signal; and a tracking error detection circuit forsubtracting said second electrical signal from said first electricalsignal to generate a tracking error signal.
 2. The apparatus of claim 1wherein said laser emits a single laser beam and said optical system hasa diffraction grating for separating said single laser beam into threebeams.
 3. The apparatus of claim 1, wherein said laser is a monolithicsemiconductor laser array that emits three laser beams from a singlelaser region.
 4. The apparatus of claim 1, wherein said laser is ahybrid semiconductor laser array having three laser regions, each ofwhich emits a single laser beam.
 5. The apparatus of claim 1, whereinsaid information recording medium rotates with a constant linearvelocity between 1.2 and 1.4 meters per second, and said grooves wobbleat a frequency of 22.05±1 kilohertz.
 6. The apparatus of claim 5,wherein said center spot is separated from each of said satellite spotsby a distance not less than 11 microns and not greater than 18 microns.7. The apparatus of claim 5, wherein said center spot is separated fromeach of said satellite spots by a distance not less than 33 microns andnot greater than 54 microns.
 8. The apparatus of claim 1, wherein saidcenter spot is distant from each of said two satellite spots bysubstantially one-fourth said pitch of said grooves in a lateraldirection of the grooves.
 9. A method for recording or reproducinginformation on or from an information recording medium having groovesthat wobble with a repeating period within a first distance, the methodcomprising:(a) supplying three beams on said information recordingmedium at an information position, a first adjacent position and asecond adjacent position, said first and second adjacent positions beinglocated an odd multiple of substantially one fourth of said firstdistance in a longitudinal direction of said grooves plus a deviation ina direction perpendicular to the longitudinal direction of said groovesfrom said information position; (b) generating a first signal from thereflected light from said first adjacent position; (c) generating asecond signal from the reflected light from said second adjacentposition; (d) determining an error signal as a function of said firstand second signals; (e) controlling the location of said informationposition based upon said error signal.
 10. The method of claim 9 whereinstep (a) includes splitting a laser beam using a diffraction grating.11. The method of claim 9 wherein step (a) includes emitting three laserbeams from a monolithic semiconductor.
 12. The method of claim 9 whereinstep (a) includes emitting three lasers from a hybrid semiconductorlaser array having three laser regions.
 13. The method of claim 9further including the step of(f) rotating said information recordingmedium with a constant linear velocity between 1.2 and 1.4 meters persecond having said grooves wobble at a frequency of 22.05 plus or minus1 kilohertz.
 14. The method of claim 9 wherein step (a) includeslinearly aligning said information position and said first and secondadjacent positions.
 15. The method of claim 14 wherein step (b) includessupplying the three beams such that said information position isseparate from said first and second adjacent positions by 11 to 18microns.
 16. The method of claim 14 wherein step (b) includes supplyingthe three beams such that said information position is separate fromsaid first and second adjacent positions by 33 to 54 microns.
 17. Themethod of claim 9 wherein said deviation is equal to one fourth thedistance between adjacent grooves.
 18. An apparatus for recording orreproducing information on or from a recording medium having groovesthat wobble with a repeating period within a first distance, theapparatus comprising:optical means for impinging three beams on saidinformation recording medium at an information position, a firstadjacent position and a second adjacent position, said first and secondadjacent positions being located an odd multiple of substantially onefourth said first distance in a longitudinal direction of said groovesplus a displacement in a direction perpendicular to the longitudinaldirection of said grooves from said information position; means fordetermining an error signal as a function of detected reflected lightfrom said first and second adjacent positions; and means for controllingthe location of said information position having said error signal as aninput.
 19. The apparatus of claim 18 wherein the optical means includesa diffraction grating.
 20. The apparatus of claim 18 wherein the opticalmeans includes a monolithic semiconductor which emits three laser beams.21. The apparatus of claim 18 wherein the optical means includes ahybrid semiconductor laser array having three laser regions each ofwhich emit a laser.
 22. The apparatus of claim 18 whereinsaid opticalmean linearly aligns said information position and said first and secondadjacent positions, and said displacement is equal to one fourth thedistance between adjacent grooves.